560 THE NERVOUS SYSTEM 



and septa-like processes which extend into the superficial portions 

 of these organs, consist of connective tissues whose fibrous bands 

 are continuous with those of the membranous pia mater. In the 

 spinal cord many of these fibrous bundles extend inward as far as 

 the grey matter. In both the spinal cord and the brain the pial 

 septa serve for the support of numerous blood vessels and perivas- 

 cular lymphatics which are distributed through this connective 

 tissue to all portions of he brain and spinal cord. 



Within the cranium, reduplications of the pia mater, carrying 

 between their folds a layer of arachnoidal tissue and an extensive 

 plexus of small blood vessels, push their way into the cerebral ven- 

 tricles to form the superior and inferior telce choroidece. These 

 choroid plexuses are separated from the ventricular cavities by an 

 investment of cuboidal cells, which in fetal and infantile life are 

 ciliated, and which are derived from and are continuous with the 

 ependyma cells lining the walls of the ventricles. Thus the blood 

 vessels of the telae choroideae, in the strictest anatomical sense, lie 

 without and not within the cavity of the cerebral ventricles, for 

 they are everywhere separated from those cavities by the ependyma 

 cells, which, ontogenetically at least, form a portion of the wall of 

 these vesicles. 



The peculiar arrangement of the three constituent membranes 

 of the meninges leaves three distinct spaces or connected groups of 

 spaces which are filled with fluid. These are the epidural, sub- 

 dural, and subarachnoidal spaces. 



The epidural space comprises a connected series of lymph cavi- 

 ties, which is of limited extent within the cranium, but of broad 

 extent within the spinal canal. These spaces are lined by endo- 

 thelium which is at many points continuous with the perivascular 

 and perineural lymphatics and through them with the lymphatic 

 vessels of the general systemic circulation. Obviously the epidural 

 spaces serve as large lymphatic vessels and their cavities are con- 

 sequently filled with lymph. 



The subdural space has a complete lining of rather thick endo- 

 thelial cells. The walls of this cavity are formed by the dura on 

 the outer, and the arachnoid on the inner side. Its cavity is occu- 

 pied by lymph and is continuous with the lymphatic vessels of the 

 dura, and through them with the epidural spaces and systemic 

 lymphatics. 



This space is penetrated by the outgoing cranial and spinal 

 nerves, which receive an investment from all three of the meningeal 



